A polypropylene resin in-mold expanded molded product, which is obtained with the use of expanded polypropylene resin particles obtained from a polypropylene resin, has characteristics such as being easily shaped, being light in weight, and being heat insulating, which are advantages of an in-mold expanded molded product. In comparison with a polystyrene resin in-mold expanded molded product which is obtained with the use of expanded polystyrene resin particles, a polypropylene resin in-mold expanded molded product is superior in terms of chemical resistance, heat resistance, strain recovery rate after compression, and the like. In comparison with a polyethylene resin in-mold expanded molded product which is obtained with the use of expanded polyethylene resin particles, a polypropylene resin in-mold expanded molded product is superior in terms of dimension accuracy, heat resistance, compressive strength, and the like. Because of these characteristics, a polypropylene resin in-mold expanded molded product is put to a wide range of use such as not only automobile interior materials and automobile bumper core materials but also heat insulating materials, shock-absorbing packing materials, and returnable containers.
As described above, a polypropylene resin in-mold expanded molded product is superior to a polyethylene resin in-mold expanded molded product in terms of heat resistance and compressive strength. However, with a polypropylene resin in-mold expanded molded product, a molding temperature during in-mold foaming molding becomes high. Therefore, a high steam pressure is necessary during, for example, in-mold foaming molding with the use of steam. This tends to cause utility costs to be high.
Certain techniques have been proposed, examples of which encompass: (i) techniques in which a low-melting polypropylene resin having a melting point of 140° C. or lower is used (e.g. Patent Literature 1), (ii) techniques in which a mixture of a low-melting polypropylene resin and a high-melting polypropylene resin is used (e.g. Patent Literatures 2 and 4-8), and (iii) techniques in which a low-melting metallocene polypropylene resin, which is polymerized by use of a metallocene catalyst, is used (e.g. Patent Literature 3). In addition to the literatures above, Patent Literatures 11 and 12 can be listed as literatures each of which discloses a technique for producing an expanded polypropylene resin particle that is excellent in characteristics such as heat resistance.
However, even though a molding temperature can be reduced with these techniques, the amount of decrease in compressive strength is excessively large in comparison with conventional in-mold expanded molded products. Specifically, for example, in a case where a polypropylene resin in-mold expanded molded product for an automobile bumper has a density of 30 g/L, a strength of approximately 0.23 MPa is required as compressive strength when the polypropylene resin in-mold expanded molded product is strained by 50% (hereinafter referred to as “50%-strained compressive strength”). With conventional technologies, a pressure of 0.26 MPa (gage pressure) or more (i.e. high molding temperature) as in-mold foaming molding pressure is necessary in order to obtain polypropylene resin in-mold expanded molded product having the strength above.
Meanwhile, in a case where (i) a low-melting polypropylene resin is used, (ii) a mixture of a low-melting polypropylene resin and a high-melting polypropylene resin is used, or (iii) a metallocene polypropylene resin, which is polymerized by use of a metallocene catalyst, is used, an in-mold expanded molded product can be molded at an in-mold foaming molding pressure of 0.20 MPa (gage pressure) or less. However, a 50%-strained compressive strength becomes considerably below 0.23 MPa. The decrease in compressive strength in a case where a polypropylene resin can be molded with such low molding pressure (low molding temperature) is remarkable when density of a molded product is 40 g/L or less.
A metallocene polypropylene resin poses a high production costs in comparison with a Ziegler polypropylene resin which is polymerized with the use of a Ziegler catalyst. Therefore, even if utility costs of in-mold foaming molding can be reduced as a result of a low molding temperature, material costs are still high. This is not necessarily advantageous from an industrial perspective.
Under the circumstances, there are still demands for a technique for achieving a high-compressive-strength polypropylene resin in-mold expanded molded product while a molding temperature during in-mold foaming molding is reduced.
As examples of a technique in which a molding temperature during in-mold foaming molding is lowered (i.e. molding at low steam pressure is enabled), there are known techniques of using expanded composite particles each including an expanded polypropylene resin core layer and a polypropylene resin covering layer that covers the polypropylene resin foamed core layer (e.g. Patent Literatures 9-10). However, these techniques tend to cause adhesiveness of an interface between an expanded polypropylene resin core layer and a polypropylene resin covering layer to be weak.